Conventional loudspeakers in widespread use employ acoustic air-driving elements of so-called "cone" type. The or each cone element is mechanically driven at its smaller end in a pistonic manner, normally by a moving coil of electromagnetic means having an operatively associated fixed magnet assembly mounted to a frame or chassis of the loudspeaker in accurate registration with the moving coil and cone assembly. Anti-phase air-excitation to rear of this assembly needs careful baffle/enclosure design to avoid cancellation effects in desired acoustic output from the front of the cone element. Naturally stiff light-weight sheet materials have been used for such cones, as well as very stiff composite sandwich structures that do not bend at all over the working frequency range; even cone elements with tailored reduction of stiffness outwardly with the aim of reducing the effective radiating area with increasing frequency to improve acoustic pistonic effects, including combating increasingly narrow directivity at high frequency. Excellent results are obtainable, e.g. using different sizes/types of cone elements and associated drive units for different frequency ranges, with appropriate electronic "cross-over" circuitry, often all in one loud-speaker housing. However, mass and bulk tend to be substantial. Moreover, sound produced is constrained by its origin with one or more cone elements whose axiality imposes unavoidably high directionality, particularly the higher the frequency; and loudness very noticeably follows the inverse square law of radiation relative to distance, as though from a point-source.
Not surprisingly, much interest and effort has long is been directed to use of flatter acoustic elements or diaphragms to occupy less space, hopefully be less directional, and preferably be less weighty. Many proposals have resulted. Some use stretched webs or films of flexible material clamped at their edges in frames, e.g. along with bonded-on current-carrying strips or wires for electro-magnetic drive using large and heavy arrays of perforated magnets, or with applied surface conduction for electro-static drive from fixed perforated polarised electrode plates requiring large high-voltage transformers and subject to loudness being limited by voltage break-down. Drive operation of these stretched film loudspeakers is inherently pistonic, and there tend to be unwanted modal "drum" and related resonances at discrete frequencies requiring specific damping provisions for satisfactory performance.
Other prior proposals have been based on using panels of expanded or foamed polystyrene edge-mounted in housings and also rely primarily on pistonic action. One example, known under the trade name `Polyplanar`, has conventional moving coil drive. Another, known as "Orthophase" has an array of magnets and coils disposed over its surface to try to achieve uniphase drive. Yet others, as available from Sound Advance Systems of California, have variously shaped flat surface polystyrene panels with complex rear ribbing and thinned edge profiling with a conventional moving coil driver mechanisms mounted to a chassis. Bertagni, from Argentina, has patented such proposals made from bonded expanded polystyrene beads, ostensibly based on how musical instruments produce sound, and requiring complex edge-clamped structures of variable thickness/flexibility, but also understood to rely basically on pistonic action. Yamaha of Japan made a large loudspeaker using a thick polystyrene diaphragm of "ear-shape" suspended at its perimeter, with moving coil drive requiring a large chassis for registering powerful magnet provision, effectively as a very large solid-cone, pistonic-action loudspeaker with a degree of self-baffling.
In a sense, these other proposals can be seen as being variations on the simple theme of almost any panel having potential for sound amplification, as long-known relative to musical boxes placed on a table top. In the 1970's, this theme was the basis for a self-contained electro-dynamic unit known as "Sonance" (see U.S. Pat. No. 3,728,497) and intended for screwing or gluing to virtually any surface, including under a table top. Not surprisingly, absence of any design control over attachment surfaces/panels, together with no better than moderate efficiency, led only to unpredictable results not satisfactory for high quality sound reproduction.